With the emergence of multimedia services such as Internet Protocol Television (IPTV) and High Definition Television (HDTV) and the growth of data services, stricter requirements are imposed on the access bandwidth. In the current access network, optical cables are replacing copper cables, and optical lines are closer to end users. Therefore, the PON is undoubtedly a preferred solution to broadband access. A PON is composed of three parts: OLT and Optical Distribution Network (ODN), which are installed in the central office, and ONU, which is installed in the user premises. In the downstream direction, the service is transmitted in broadcast mode; and in the upstream direction, multiple ONUs access the network in Time Division Multiplexing (TDM) mode.
Currently, the PON system is categorized into the following types: ATM Passive Optical Network (APON) whose transmission platform is based on the Asynchronous Transfer Mode (ATM), Ethernet Passive Optical Network (EPON) whose transmission platform is based on the Ethernet technology, and Gigabit Passive Optical Network (GPON) whose transmission platform is based on a universal frame structure. The frame structure of a PON (taking the GPON as an example) is described below.
As shown in FIG. 1, in a GPON, the downstream frame is a 125 μs frame structure, and includes a Physical Control Block downstream (PCBd) overhead area and a payload area. The PCBd overhead area includes a Physical Synchronization (PSync) field, a super frame indication (Ident) field, a Physical Layer Operations, Administration and Maintenance downstream (PLOAMd) field, a Bit Interleaved Parity (BIP) field, a Payload Length downstream (PLend) field, and an Upstream Bandwidth Map (US BW Map) field. The PLOAMd field carries downstream Physical Layer OAM (PLOAM) information.
FIG. 2 shows the upstream frame structure in a GPON. Each ONU sends upstream burst packets to the OLT in the T-CONT allocated by the OLT to the ONU. Each of the upstream burst packets includes an overhead area and a payload area. The overhead area includes a Physical Layer Overhead upstream (PLOu) field, a Physical Layer OAM upstream (PLOAMu) field, a Physical Layer Sequence upstream (PLSu) field for adjusting power, and a Dynamic Bandwidth Report upstream (DBRu) field. The PLOAMu field carries upstream PLOAM information.
Therefore, a PLOAM field exists in both the upstream and the downstream frame structures in the GPON. As shown in FIG. 3, the PLOAM frame structure includes: an ONU identifier (ONU-ID), a Message-ID, a message, and a Cyclic Redundancy Code (CRC). There are 19 message types in the upstream direction and 9 message types in the downstream direction.
In the existing PON system, only unicast and broadcast mechanisms are available. The unicast mechanism means the communication between a single ONU and an OLT. The broadcast mechanism means the communication between all ONUs and an OLT, and the communication is performed by broadcasting 255 in the ONU-ID. The transmission of the existing video service is described below with reference to FIG. 4.
FIG. 4 shows a PON system for transmitting video services. Table 1 specifies the ONU-ID and the Port-ID. If the same data needs to be sent to the ONU1, ONU3, ONU4, and ONU5, the OLT needs to have four copies of such data, and encapsulate the data into six GPON Encapsulation Method (GEM) frames, with the Port-IDs being 11, 13, 14, and 15, respectively. The GEM frames are sent to different ONUs at different time based on the TDM principle.
TABLE 1Mapping between ONU-ID and Port-IDONU IDPort ID111212313414515616
Moreover, the distance from one ONU to one OLT is unequal to the distance from another ONU to the OLT. Therefore, the transmit power of the ONU needs to be adjusted to ensure that the upstream optical power at the OLT falls within the receiving range of the OLT. The PON system performs the adjustment for each ONU separately. In a practical system, the OLT receiver covers a specific scope, and some ONUs (for example, the ONU1, ONU3, ONU4, and ONU5) are close to each other, and may be regarded as a group for being adjusted uniformly. However, this is impossible in the prior art. The prior art can only adjust each ONU at different time in TDM mode.
In the process of implementing the present disclosure, the inventor finds at least the following defects in the prior art:
The existing PON supports only unicast and broadcast. For multiple ONUs that use the same service, the same data needs to be sent to each ONU. Consequently, the same data is sent repeatedly in the network, thus wasting bandwidth resources.
It is impossible to perform unified management and adjustment for a group of ONUs, and each ONU needs to be adjusted separately, thus leading to low efficiency.
In the existing PON system, no relevant commands are specified for multicast. With the development of the Video On Demand (VOD) service, multicast becomes one of the important functions of a PON. The “long-reach and large-split-ratio” PON system is put forward, where the quantity of ONUs in a PON is greater than the existing 128 and is expected to exceed 1000. Under this background, the multicast management for various management groups is essential.